This invention relates generally to a method and apparatus for recording a digitized video signal on a magnetic tape and, more particularly, is directed to a method and apparatus for recording a digitized video signal on a magnetic tape with a high recording density.
Conventionally, apparatus for recording a video signal on a magnetic tape have been of the analog, rather than digital, type. However, there has been a recent turn towards development of digital video tape recorders (VTR). Digital VTRs have a very high picture quality, which enables multiple generation dubbing with virtually no picture impairment. Further, digital VTRs provide adjustment free circuits and self-diagnostic systems which enable easier maintenance and higher reliability.
With digital VTRs, an analog video signal is converted into digital form by an A/D (analog-to-digital) converter. In particular, the analog video signal is sampled by clock pulses having a sampling frequency which may be, for example, 4f.sub.sc, where f.sub.sc is the color sub-carrier frequency of the color video signal, resulting in the analog video signal being converted into a digitized video signal comprised of 8-bit words. The digitized signal is also coded by an error control encoder so that errors may be corrected and concealed on playback and, it is further coded by a channel encoder to achieve high density digital recording. The coded digitized signal is then recorded on a magnetic tape by means of a recording amplifier. However, it should be appreciated from the above that the recording bit rate, that is, the rate of occurrence of each bit of the digitized video signal, is extremely high. For example, in the above-described embodiment, where the color sub-carrier frequency f.sub.sc =3.58 MHz, the recording bit rate is equal to 4f.sub.sc times the number of bits per word. In other words, the recording bit rate is obtained as follows: EQU Bit rate=4.times.3.58.times.10.sup.6 .times.8=114.6 Mb/s.
Because of such high recording bit rate, the digitized video signal is not suitable for recording in a single recording channel.
Accordingly, it has been proposed to separate the digitized video signal into at least two separate channels prior to recording it on a magnetic tape so as to reduce the recording bit rate per channel. Typically, a magnetic head is associated with each channel and all of the magnetic heads are aligned to record the respective channels on a magnetic tape in parallel tracks extending obliquely on the tape. In order to separate the digitized video signal into, for example, two channels, an interface is provided which distributes alternate 8-bit words of the digitized video signal into the respective channels.
In recording the digitized video signal in the parallel tracks, it is desirable to increase the signal-to-noise (S/N) ratio so that, during reproduction, a video picture of high quality can be obtained, while at the same time, reducing the amount of tape consumption by recording the digitized signal with a high density. It should be appreciated that these two objections are contrary to one another. For example, as the track width is decreased so as to obtain such high density recording, the S/N ratio of the video signal reproduced from the tracks deteriorates. In like manner, as the track width is increased which results in higher tape consumption, the S/N ratio increases.
Therefore, in such previously proposed apparatus, guard bands have been provided between adjacent ones of the parallel tracks recorded on the magnetic tape so as to avoid cross-talk interference between such adjacent tracks, resulting in a higher S/N ratio. However, when the track width is 40 .mu.m, for example, the width of the guard band between each of the adjacent tracks must be at least 20 .mu.m, resulting in a high tape consumption. If, on the contrary, the track width is made narrower, tracking errors are apt to occur during the reproduction operation, wherein the heads do not accurately trace the recorded tracks, resulting in a deterioration of the S/N ratio. Also, with a reduction of track width, there necessarily is a reduction in the width of the guard bands, resulting in increased cross-talk interference (noise) from adjacent tracks.